1. Field of the Invention
The present invention relates to an optical pickup apparatus, and more particularly, to an optical pickup apparatus using a double reflection polarizing plate.
2. Description of the Prior Art
In general, optical pickup apparatuses that are used in compact disc reproduction apparatuses and the like use a diffraction device or a holographic optical element so that the number of components of the optical pickup apparatus can be reduced. The optical pickup apparatuses that are used in the compact disc reproduction apparatuses and the like employ a three spot method, in which two sub-beams for detecting a tracking error in addition to the main beam are used, in order to detect a tracking error signal. An example of techniques using the above three spot method is disclosed in U.S. Pat. No. 5,128,914.
FIG. 1 is a side view for illustrating a conventional optical pickup apparatus. The optical pickup apparatus includes a diffraction device 2, an optical system and a light element 7 including at least one of photodetectors and a light source 1. In the above optical pickup apparatus, a laser beam A emitted from light source 1 is diffracted through diffraction device 2 into three-separated light beams. One of the three-separated light beams is a zero-order diffracted beam A1, which is focused on a recording medium 5 through the optical system as a main beam. The others of the three-separated light beams which correspond to two sub-beams A2 and A3 obtained by means of the above three spot method are two first-order diffracted beams which have respectively different directions, i.e., positive and negative directions. The two first-order diffracted beams A2 and A3 are positioned symmetrically to the main beam and shift to the tracking direction to be focused on the recording medium. The beams reflected on recording medium 5 are diffracted by diffraction device 2 to make the first-order diffracted beams, which are focused on a photodetector 6. Regions 2a, 2b and 2c of the diffraction device which produce the sub-beams A2 and A3 of the three spot method have brazing characteristics so as to strengthen the light intensity of the first-order diffracted beam alone that proceeds in the positive direction, a sufficient sensitivity being obtainable by means of light element 7 wherein light source 1 and photodetector 6 are integrated instead of plural photodetectors. Light element 7 is divided into at least three regions in the same way as that of the well known three spot method. The regions are irradiated with the corresponding beams diffracted by the diffraction device. The corresponding beams resulting from diffraction are focused on light element 7 to produce an output signal of light element 7. A focus signal is obtained by the output signal by means of a knife edge method. A tracking error signal can be detected based on the three-spot method from a difference between the light intensities of the diffracted beams resulting from the sub-beams A2 and A3. The tracking error signal according to the prior art can be detected based on the three spot method, and thus an offset never arises in the tracking error signal even when the optical axis of the optical system is displaced from a given position. Moreover, because of the use of a diffraction device, the number of components of the optical system can be reduced. Further, light source 1 and photodetector 6 are incorporated into one body by disposing them within a package and the diffraction device is used as a window that seals the package, so that the production cost of the optical pickup apparatus can be lowered.
However, since the optical pickup apparatus using the diffraction device detects the error signal by employing the diffracted beams, in case that the recording medium such as an optical disc is slantingly positioned or there are some defects on the surface of the recording medium, the detection signal would change or deviate from the given position, so that the photodetector can not help erroneously detecting the signals. On the contrary, an optical pickup apparatus using a double refraction polarizing plate according to the present invention detects the polarized beams, so that the optical pickup apparatus can remove detection errors resulting from some defects made on the surface of the recording medium.